1. Field of the Invention
The present invention relates to an optical fiber for a wavelength division multiplexing (WDM) optical transmission system, and more particularly to an optical fiber capable of suppressing influences, caused by a non-linearity of optical fibers, to a maximum in order to obtain a maximum transmission capacity per optical fiber. Also, the present invention relates to an optical fiber which can operate efficiently even when a reduced channel spacing is used for an increase in transmission capacity, while being usable even in a wavelength band of 1,450 to 1,530 nm called an xe2x80x9cS-bandxe2x80x9d expected to be used in WDM optical transmission systems in future.
2. Description of the Related Art
Optical fibers can transmit a large quantity of data within a short period of time while involving a reduced transmission loss. The use of such optical fibers has been greatly increased in accordance with recent development of communications. In particular, optical transmission techniques have been remarkably advanced by virtue of development of a new optical fiber capable of transmitting signals for a long distance while involving a reduced signal loss, and development of a superior light source such as a semiconductor laser. In pace with such development of optical transmission techniques, techniques associated with optical fibers has been greatly advanced.
However, known optical fibers involve a chromatic dispersion, that is, a phenomenon that a signal is spread due to a difference in group velocity inverse to a variation in phase constant for different wavelength components of the signal, that is, different mode frequencies. Due to such a chromatic dispersion, a signal overlap occurs at the receiving terminal, thereby resulting in a fatal problem such as an impossibility of demodulation. For this reason, attempts to minimize such a chromatic dispersion (hereinafter, simple referred to as a xe2x80x9cdispersionxe2x80x9d) have been made. By virtue of such attempts, it has been found that a zero dispersion is achieved at an operating wavelength of 1,310 nm.
Meanwhile, it has been found, on the basis of the relation between the total loss and the wavelength in an optical fiber, that a minimum signal loss is exhibited at a wavelength of 1,550 nm even though an increased dispersion occurs, as compared to that occurring at 1,310 nm. In this connection, the operating wavelength of 1,550 nm could be used by virtue of the development of a new optical amplifier capable of amplifying the wavelength band of 1,530 nm to 1,565 nm. As a result, a non-repeating long distance transmission has been possible. This has resulted in the advent of a dispersion-shifted fiber (DSF) adapted to shift the zero dispersion from the wavelength of 1,310 nm, at which the zero dispersion is achieved in conventional cases, to the wavelength of 1,550 nm in order to obtain a minimum dispersion and a minimum signal loss.
In addition to such a development of optical fibers, a WDM system has been developed, which serves to multiplex a plurality of optical signals having different wavelengths so as to simultaneously transmit those optical signals through a single optical fiber. Using such a WDM system, it is possible to more rapidly transmit an increased amount of data. An optical communication system using a WDM scheme at a wavelength of 1,550 nm has already been commercially available.
Where the above mentioned DSF is used in such a WDM optical transmission system, however, a signal distortion may occur even though a desired zero dispersion may be achieved. This is because the zero dispersion in the optical fiber may result in a non-linearity of the optical fiber, for example, a four-wave mixing in which lights of different wavelengths may be mixed together.
The most practical method usable in the WDM optical transmission system for a further increase in transmission capacity is to increase the number of channels used. In order to increase the number of channels used, however, it is necessary to use a reduced channel spacing because optical amplifiers use a limited amplification band. Such a reduced channel spacing may result in a more severe problem associated with the non-linearity of the optical fiber such as the four-wave mixing. The non-linearity of an optical fiber becomes more severe at a reduced channel spacing or a decreased dispersion of the optical fiber.
U.S. Pat. No. 5,327,516 discloses an optical fiber for a WDM system which exhibits a dispersion ranging from 1.5 ps/nm-km to 4 ps/nm-km at a wavelength of 1,550 nm in order to achieve a suppression in non-linearity. The optical fiber disclosed in this patent is called a xe2x80x9cnon-zero dispersion-shifted fiber (hereinafter, referred to as an xe2x80x9cNZ-DSFxe2x80x9d) in that it is configured to obtain a non-zero dispersion. Such an optical fiber is commercially available from Lucent Technologies In., U.S.A.
The NZ-DSF is significant in that it can suppress the four-wave mixing phenomenon by virtue of its dispersion value ranging from 1.5 ps/nm-km to 4 ps/nm-km. However, the NZ-DSF disclosed in U.S. Pat. No. 5,327,516 insufficiently suppresses the four-wave mixing phenomenon occurring in current WDM systems using a channel spacing reduced from 200 GHz to 50 GHz via 100 GHz. For this reason, it is difficult for this NZ-DSF to be applied to a WDM long-distance optical transmission system using a narrow channel spacing of about 50 GHz.
FIG. 1 schematically illustrates an example of a WDM optical transmission system using NZ-DSFs.
The optical fiber system of FIG. 1 has 8 channels with a channel spacing of 50 GHz. This optical fiber system, which is denoted by the reference numeral 10, receives optical power of 0 dBm per channel from a light source. NZ-DSFs 14 are distributed over a total distance of 480 km. A dispersion compensation optical fiber (DCF) 15 is also arranged in every span, along with an optical amplifier 13. The detailed specification of the optical transfer system 10 illustrated in FIG. 1 is described in the following Table 1.
The optical transmission system of FIG. 1 mainly includes eight transmitters (Tx) 11 respectively adapted to provide lights of different wavelengths, a multiplexer for multiplexing the lights of different wavelengths transmitted from the transmitting terminals 11, a plurality of optical amplifiers 13 each adapted to amplify a multiplexed light outputted from the multiplexer, a plurality of DCFs 15 each adapted to compensate for an amplified light outputted from an associated one of the optical amplifiers 13 arranged just upstream from the DCF 15, a demultiplexer for demultiplexing the light finally outputted after passing through the optical amplifiers 13 and DCFs 15, and a receiver (Rx) 12 for receiving the demultiplexed light from the demultiplexer. A plurality of NZ-DSFs 14 are distributed between the transmitters 11 and receiver 12. The optical amplifiers 13 are arranged so that each of them is spaced apart from an associated one of the NZ-DSFs 14 by a desired distance.
Each of the NZ-DSFs 14 used in the optical transmission system of FIG. 1 exhibits an average dispersion of 3.0 ps/nm-km. The average dispersion is a value obtained by dividing a dispersion value accumulated during the transmission of an optical signal by a transmission distance. Each NZ-DSF 14 exhibits an accumulated dispersion value of about 240 ps/nm at a point of 80 km. This accumulated dispersion value of each NZ-DSF 14 is compensated for by an associated one of the DCFs 15 each having a dispersion value of xe2x88x92240 ps/nm.
FIG. 2a is an eye diagram of an optical signal transmitted in the optical transmission system illustrated in FIG. 1.
As apparent from FIG. 2a, the eye of the optical signal is unclear, and partially opened. That is, the optical signal is in a severely degraded state. Such a signal degradation is mainly caused by a four-wave mixing phenomenon.
FIG. 2b illustrates the optical spectrum of an optical signal transmitted in the optical transmission system of FIG. 1.
Referring to FIG. 2b, it can be found that a signal spectrum not associated with the transmitted optical signal is generated at portions of the optical signal indicated by the arrow 35. Such a signal spectrum is generated due to a four-wave mixing phenomenon. Where a WDM optical transmission system using NZ-DSFs uses a narrow channel spacing of 50 GHz, its transmission quality is severely degraded due to a four-wave mixing phenomenon. This can be found by referring to FIG. 2b. 
FIG. 3 illustrates respective variations in the Q-value, indicative of the communication quality, in the transmission system of FIG. 1 depending on the optical power inputted per channel at a channel spacing of 50 GHz in the cases where conventional NZ-DSFs are distributed over a distance of 320 km and a distance of 640 km, respectively.
It is generally known that when the Q-value is 16 dB or more, a bit error rate of 10xe2x88x929 or less is obtained, at which there is no interference with communications. Referring to FIG. 3, it can be found that where the input optical power per channel generally used at 10 Gb/s is 3 dBm, the transmission system exhibits a Q-value of 16 dB or less in a long-distance transmission. That is, there is a severe signal degradation.
FIG. 4 illustrates an experimental system in which an experiment is conducted under the condition in which the input optical power per channel is 12 dBm, in order to determine a variation in four-wave mixing level depending on the channel spacing.
For the experiment conducted by the experimental system of FIG. 4, two optical fibers were used, one of which is a conventional NZ-DSF exhibiting a dispersion of 2.5 ps/km-nm at a wavelength of 1,550 nm while the other optical fiber being an optical fiber having characteristics described in the following Table 2. In the experiment, respective levels of four-wave mixing generated in the two optical fibers at various channel spacings are compared with each other.
FIG. 5a is a graph depicting a four-wave mixing generated in the conventional NZ-DSF at a channel spacing of 25 GHz with respect to a certain frequency band.
FIG. 5b is a graph depicting a four-wave mixing generated in the optical fiber increased in dispersion, as shown in Table 2, to reduce the four-wave mixing, with respect to the same frequency band as that in the case of FIG. 5a. 
Referring to the comparison between FIGS. 5a and 5b, it can be found that the case of FIG. 5b exhibits a considerable reduction in the signal distortion resulting from the four-wave mixing (portions indicated by xe2x80x9cFWMxe2x80x9d in FIGS. 5a and 5b).
FIGS. 6a and 6b are graphs respectively depicting results obtained after the same experiments as those in the cases of FIGS. 5a and 5b are conducted at a channel spacing of 50 GHz.
Referring to the comparison between FIGS. 6a and 6b, it can be found that the case of FIG. 6b exhibits a considerable reduction in the signal distortion resulting from the four-wave mixing (portions indicated by xe2x80x9cFWMxe2x80x9d in FIGS. 6a and 6b).
Referring to FIGS. 5a, 5b, 6a, and 6b, it can be found that the conventional NZ-DSF inefficiently suppresses the four-wave mixing. It can also be found that the dispersion should be at least 8 ps/km-nm for an efficient suppression of the four-wave mixing.
FIG. 7 is a graph depicting variations in signal-to-noise ratio exhibited in respective cases using the conventional NZ-DSF and a 50 GHz NZ-DSF, depending on a variation in four-wave mixing resulting from a variation in channel spacing.
As shown in FIG. 7, the 50 GHz NZ-DSF effectively suppresses the four-wave mixing, as compared to the conventional NZ-DSF. In FIG. 7, each dot represents an experimental value whereas each line is a theoretical value. Referring to FIG. 7, it can be found that there is a considerable difference between the experimental and theoretical values. This difference results from noise other than the four-wave mixing. Where the channel spacing used in a WDM optical transmission system using the conventional NZ-DSF is reduced to 50 GHz for an increase in transmission capacity, therefore, it is impossible to transmit optical signals at a desired transmission quality due to a four-wave mixing phenomenon occurring due to the reduced channel spacing. This result causes a limitation on the maximum transmission capacity of the optical transmission system using the NZ-DSF. In this regard, where a WDM optical transmission system is desired to reduce the channel spacing to 50 GHz for an increase in the transmission capacity per optical fiber, it is important to develop an optical fiber having suppression characteristics for the four-wave mixing phenomenon, as compared to conventional NZ-DSFs.
The wavelength band mainly used in current WDM optical transmission systems is a wavelength range of 1,530 to 1,565 nm called a xe2x80x9cC-bandxe2x80x9d. Also, the wavelength range of 1,565 to 1,610 nm called a xe2x80x9cL-bandxe2x80x9d is often used. One method for allowing optical transmission systems to have a large-scale capacity is to expand the operating wavelength range. For such an expansion of the operating wavelength range, it is necessary to develop an amplifier capable of amplifying light of a desired wavelength range. Currently, active research is being made in association with amplifiers capable of amplifying light of the S-band, that is, the wavelength range of 1,450 to 1,530 nm. It is known that optical amplifiers using an optical fiber made of glass based on fluoride and tellurite, and Raman amplifiers using a Raman scattering effect generated in an optical fiber are usable in the S-band.
Conventional NZ-DSFs cannot achieve an easy WDM optical transmission in the S-band because they exhibits a zero dispersion in the S-band, thereby resulting in an occurrence of the four-wave mixing phenomenon. To this end, it is necessary to develop an optical fiber having a certain dispersion value in the S-band so as to enable a WDM optical transmission even in the S-band, thereby meeting requirements involved in future optical transmission systems.
In order to allow optical transmission systems to have a large-scale capacity, therefore, it is urgently necessary to develop optical fibers capable of supporting both the 50 GHz channel spacing and the S-band, which cannot be simultaneously supported by conventional NZ-DSFs.
Therefore, an object of the invention is to solve the above mentioned problems involved in the related art, and to provide an optical fiber capable of being applied to a WDM optical transmission system using a reduced channel spacing while maximizing the transmission capacity per optical fiber in the WDM optical transmission system.
Another object of the invention is to provide an optical fiber having an appropriate dispersion value to sufficiently suppress a non-linearity phenomenon, that is, a four-wave mixing phenomenon, occurring at a channel spacing of 50 GHz while minimizing the expense consumed for a compensation for dispersion.
Another object of the invention is to provide an optical fiber exhibiting a zero dispersion wavelength of 1,450 or less, thereby being capable of achieving a WDM optical transmission in the S-band.
Another object of the invention is to provide an optical fiber exhibiting a desired dispersion value at a wavelength of 1,300 nm, expected to be used following the S-band, and a cut-off wavelength of 1,200 nm or less, thereby being capable of achieving a WDM optical transmission at the wavelength of 1,300 nm.
In order to accomplish these objects, the present invention provides an optical fiber for a wavelength division multiplexing optical transmission system using a channel spacing of 50 GHz, wherein the optical fiber satisfies optical characteristics defined by a dispersion value of 7 to 10 ps/nm-km at a wavelength of 1,550 nm, a zero dispersion wavelength of 1,450 nm or less, and a cut-off wavelength of 1,250 nm or less, and comprises a core having a desired diameter (d1) and a desired refractive index (n1), and a cladding made of a pure silica glass, the cladding surrounding the core and having a refractive index (ncl) less than the refractive index (n1) of the core (nc1 less than n1).
An inner cladding having a desired diameter (d2) may be interposed between the optical fiber and the cladding.
The inner cladding may have a refractive index (n2) less than the refractive index (ncl) of the outer cladding (n2 less than ncl). In this case, the ratio between the diameter (d1) of the core and the diameter (d2) of the inner cladding (d1/d2) ranges from 0.3 to 0.8.
Alternatively, the refractive index (n2) of the inner cladding may be less than the refractive index (n1) of the core while being more than the refractive index (ncl) of the outer cladding (nc1 less than n2 less than ncl). In this case, the diameter ratio (d1/d2) ranges from 0.35 to 0.7.